Communication with electromagnetic waves between a transmitter and a receiver is very common. Frequently, especially with shorter waves, communication is limited to line-of-sight and the horizon is the natural limit to how far such communications can be made between a transmitter and a receiver. In such cases, the distance of transmission can be extended by means such as putting the transmitter's antenna on a high hill, transmitting to one or more receivers which are equipped to retransmit to other receivers, or transmitting via cable which carries transmissions to an antenna within line-of-sight of receivers. The most successful means to extend the horizon for a transmission of electromagnetic waves is to transmit them to a communications satellite whicn in turn retransmits back to earth. A single satellite located high above the earth has a transmission horizon that is substantially half the surface of the earth. Many such communication satellites now exist.
Communication satellites largely are in substantially the same orbit and they are placed in orbit at a height and speed such that they hover above a specific spot on earth. To receive communications retransmitted from a satellite it is necessary to use a highly directional antenna and to aim it accurately toward the satellite. In addition to aiming the antenna accurately toward the satellite whose communications are desired, it is also necessary to locate the directional antenna so that no solid object such as a structure or a mountain or even a large tree is between the antenna and the satellite.